Sensing the presence and/or concentration of a gas in a local environment or atmospheric region can employ a technology known generally as gas correlation. More specifically, gas filter correlation radiometry (GFCR) provides a method of creating a signal that is highly correlated (and highly filtered) to a specific gas, and therefore a good measure of that gas. However, for each gas measured, GFCR requires either a split beam pair or a gas-modulated beam filtered for that target gas. Thus, to apply GFCR to multiple gases, the instrument can become large, optically complex and expensive as the number of beam splitters increases.